1. Field of the Invention
The present invention relates to a turbo-molecular pump for evacuating gas in a chamber used in a semiconductor fabrication process or the like, and more particularly to a turbo-molecular pump which is compact and has a high evacuating capability.
2. Description of the Related Art
Processes of fabricating high-performance semiconductor devices employ a turbo-molecular pump for developing high vacuum or ultrahigh vacuum. The turbo-molecular pump comprises a rotor rotatably supported in a cylindrical casing and having a plurality of rotor blades, the cylindrical casing having a plurality of stator blades projecting from an inner surface thereof between the rotor blades. The interdigitating rotor and stator blades make up a turbine blade pumping assembly. When the rotor is rotated at a high speed, gas molecules move from an inlet of the cylindrical casing to an outlet thereof to develop a high vacuum in a space that is connected to the inlet.
In order to achieve a high vacuum, it is necessary for the pump to provide a large compression ratio for the gas. Conventional efforts to meet such a requirement include providing the rotor and stator blades in a multistage manner or incorporating a thread groove pumping assembly downstream of the turbine blade pumping assembly. The rotor and a main shaft supporting the rotor are supported by magnetic bearings for easy maintenance and high cleanliness.
Recently, semiconductor fabrication apparatuses tend to use a larger amount of gas as wafers are larger in diameter. Therefore, a turbo-molecular pump used to evacuate gas in a chamber in such a semiconductor fabrication apparatus is required to evacuate gas in the chamber at a high rate, keep the chamber under a predetermined pressure or less, and have a high compression capability.
However, the turbo-molecular pump capable of evacuating gas in the chamber at a high rate and having a high compression capability has a large number of stages, a large axial length, and a large weight, and is expensive to manufacture. In addition, the turbo-molecular pump takes up a large space around the chamber in a clean room. Such space needs a large construction cost and maintenance cost. Another problem is that when the rotor is broken under abnormal conditions, the turbo-molecular pump produces a large destructive torque, and hence cannot satisfy desired safety requirements.
It is therefore an object of the present invention to provide a turbo-molecular pump which is axially compact and has a sufficient evacuation and compression capability.
In order to achieve the above object, according to the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing; a rotor supported in the casing and being rotatable at a high speed; and a turbine blade pumping assembly and a thread groove pumping assembly which are disposed between the stator and the rotor; the rotor being formed by joining at least two components which are separable from each other at a predetermined position. The rotor comprises at least two components that are axially separate from each other.
The components of the rotor can individually be manufactured by machining, for example. The rotor can easily be manufactured under less strict machining limitations so as to have a shape suitable for a high evacuation and compression capability. Therefore, the turbo-molecular pump can evacuate gas at a high rate and has high compression capability.
The thread groove pumping assembly may comprise at least one of a spiral thread groove pumping assembly for discharging gas molecules radially and a cylindrical thread groove pumping assembly for discharging gas molecules axially. A plurality of cylindrical thread groove pumping assemblies may be radially superposed to provide a passage of increased length for discharging gas molecules.
The components of the rotor can be joined by shrink fitting or bolts. If the components of the rotor have interfitting recess and projection, then the components can easily be positioned with respect to each other and firmly be fixed to each other. The position where the components of the rotor are separable from each other is determined in consideration of simplicity for manufacturing the rotor and the mechanical strength of the rotor. For example, the components of the rotor may be separate from each other between the turbine blade pumping assembly, and the spiral thread groove pumping assembly or the cylindrical thread groove pumping assembly.
The spiral thread groove pumping assembly is usually disposed downstream of the turbine blade pumping assembly, and has evacuating passages for discharging gas molecules in a radial direction. Therefore, the spiral thread groove pumping assembly has an increased evacuation and compression capability without involving an increase in the axial dimension thereof. Although the rotor with the spiral thread groove pumping assembly is complex in shape, the rotor can be manufactured with relative ease because it is composed of at least two components which are separable from each other.
The cylindrical thread groove pumping assembly is usually disposed downstream of the turbine blade pumping assembly, and provides a cylindrical space between the rotor and the stator. The cylindrical thread groove pumping assembly may be arranged to provide two or more radially superposed passages for discharging gas molecules. The cylindrical thread groove pumping assembly having the above structure provides a long passage for discharging gas molecules, and has an increased evacuation and compression capability without involving an increase in the axial dimension thereof. Although the rotor with the cylindrical thread groove pumping assembly is complex in shape, the rotor can be manufactured with relative ease because it is composed of at least two components which are separable from each other.
The components of the rotor may be made of one material or different materials. Blades of the stator and rotor may be made of an aluminum alloy. However, when the turbo-molecular pump operates under a higher back pressure than the conventional one, the components made of the aluminum alloy tend to suffer strains caused by forces or pressures applied to the rotor or creep caused by increase of temperature, resulting in adverse effects on the stability and service life of the pump. In addition, the rotor may rotate unstably because the components of the aluminum alloy are liable to be expanded at higher temperatures. According to the present invention, some or all of the components of the rotor may be made of a titanium alloy which has a high mechanical strength at high temperatures or ceramics which have a high specific strength and a small coefficient of thermal expansion. The components made of the titanium alloy or ceramics are prevented from being unduly deformed or thermally expanded to reduce adverse effects on the service life of the pump and to operate the pump stably. These materials are also advantageous in that they are highly resistant to corrosion. Furthermore, because the rotor is composed of at least two components, the rotor may be made of one or more of different materials depending on the functional or manufacturing requirements for the pump.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.